1. Field of the Invention
The present invention concerns an improvement made in the structure of a semiconductor laser so as to improve the localization of the current that flows through the emissive strip of the laser.
2. Description of the Prior Art
It is known that a semiconductor laser includes at least one light-emitting layer, called an active layer, placed between two confinement layers. In the active layer, a strip is defined by etching: it is this layer which, in forming a Fabry-Perot cavity, emits a coherent light beam through its two cleaved faces at the ends of the strip.
The current injected from a contact layer, adjacent to a confinement layer, towards the substrate, adjacent to another confinement layer, creates the emission of light by the active layer. One of the problems of light-emitting semiconductor devices is that of localizing the current injection in the region of the strip alone, i.e. limiting it to this region.
A known approach consists in defining the injection of current in the layer by means of a reverse biased junction which stretches around the active strip and thus circumscribes it and counters the passage of the current. However, the making of a junction, during fabrication, calls for an additional epitaxy (by localized growth) and this considerably complicates the making of the device.
Another simple known approach consists in making two parallel diodes, one homojunction diode and one heterojunction diode, with the difference in the voltages corresponding to the flexion point on the characteristic curve enabling the injection of current preferably in the diode with the lower threshold voltage. It is therefore the heterojunction diode, formed between two confinement layers and the active layer, that favors the passage of the current through the etched strip. This is valid for a low current but, at high currents, the current flowing in the homojunction diode is no longer negligible. Furthermore, the efficiency of the confinement of the current is diminished when the temperature rises.
This is illustrated by the FIGS. 1, 2 and 3 pertaining to the prior art.
FIG. 1 is deliberately limited to that region alone of a laser which is necessary for understanding the operation: the substrate, the auxiliary layers and the biasing metallizations are not shown.
Let us take an emissive strip 1 between a first layer of confinement 2, made of InP with P type doping, and a second confinement layer 3, made of InP with N type doping, for example. In this case, the strip 1 is made of GaInAsP, for example. The diode with low threshold voltage is formed by the heterojunction 2-1-3 formed by InP, GaInAsP and InP, and the diode with the higher threshold voltage is formed by the homojunction 2-3 between P type and N type InP.
The current that flows through the layers goes through the heterojunction diode (channel A) and the homojunction diode (channel B). The equivalent electrical diagram is given in FIG. 2: the resistance of access R.sub.2 to the homojunction diode D.sub.2-3 is substantially the same as the resistance of access R.sub.1 to the heterojunction diode D.sub.2-1. Only the voltages corresponding to the flexion point of their characteristic curves are different. If it is desired to increase R.sub.2 by lower doping of the semiconductor material on the channel B, R.sub.1 is also increased because the semiconductor material is less doped at 4, on the channel A of access to the strip 1, and the leakage current of the homojunction is increased.
The curves of V(I) for these two diodes are given in FIG. 3: at high currents, the leakage current through the channel B is excessive as compared with the current in the channel A. The curves of the two diodes are substantially parallel to each other.
The invention provides an improvement to this device for the localization of the current injected into the emissive strip, by creating a preferred path for the current, by means of a localized diffusion of zinc, right on the strip, in a semiconductor confinement layer which itself has a doping gradient that decreases with distance from the emissive strip.